It is difficult to permanently bond copper surfaces and insulating organic substrates together. Copper has a tendency to form a weakly adherent oxide layer on its surface. Organic films may bond tenaciously to this oxide layer, but the layer in time separates from the copper surface causing failure of the bond.
In the printed wiring board industry, application of strongly adherent oxide layers on copper has been adopted to alleviate this problem. Such strongly adherent oxide layers are usually applied by immersing the copper surface in hot (40.degree.-110.degree. C.), strongly alkaline, hypochlorite solutions. This immersion produces an adherent, black, dendritic, oxide layer with a high surface area for adhering to organic films, coatings and laminated layers. In the printed wiring industry, this oxide layer is commonly called "black oxide".
Black oxide has not been completely problem free. If the immersion in the hot, alkaline, hypochlorite solution is too long or too hot, the oxide film may build too thick and the oxide dendrites become so long that they easily break off. This would lead to bond failure.
The black oxide layer is subject to attack by solutions which dissolve copper oxides. Such solutions include acids, and acidic cleaners, and solutions containing complexing and chelating agents for copper such as electroless and electroplating solutions. Use of such solutions are necessary in printed wiring board, and especially in multilayer board manufacturing. In multilayer board manufacturing, inner copper planes are coated with black oxide, and outer layers of insulator and copper are laminated over them. When holes are drilled through the multilayer laminate and the hole walls are plated to create electrical connections to the inner copper planes, the plating and cleaning solutions dissolve the black oxide surrounding the holes and leave a ring around the hole where there is no adhesion between the copper plane and the laminated insulating layer over it. This is known in the industry as "pink ring" since a pink ring of copper is visible in pattern of black oxide coated copper.
The black oxide layer is mainly copper (II) oxide. At elevated temperatures, copper (II) oxide is thermodynamically unstable and converts to copper (I) oxide destroying the bond between the organic layer and the copper surface. Also, at elevated temperatures, many organic compounds are oxidized by the copper oxide layer and, in turn, reduce the oxide to copper metal. These two effects combine to make the black oxide bond subject to failure under long term thermal aging conditions.
Many variations such as "thin oxide", "brown oxide" and "red oxide" have been offered to overcome the problems of black oxide. None has been universally successful. Black oxide has been completely replaced by a brass coating in the lamination of copper foil to printed wiring board base materials. The copper foil is coated on one side with a thin layer of electroplated brass or zinc and the coated layer bonded to the organic insulating base material. Some multilayer board manufacturers have used copper foil coated on both sides with brass to avoid using black oxide when bonding the outer layers to the copper surface of the inner layers. Unlike black oxide, the layer of brass cannot be applied after the inner layer conductor pattern has been etched, and the thin brass layer is easily damaged by handling, cleaning and printing and etching of the inner layer.
Poly(vinyl acetals), poly(vinyl formal) and poly(vinyl butyral), have been blended with phenolic resins for use as wire coatings and as adhesives for bonding metal to metal, for brake shoes and for honeycomb construction. Poly(vinyl butyral)-phenolic adhesives had been used for laminating copper foil to printed wiring board base materials. This use of poly(vinyl butyral)-phenolic adhesives was abandoned in the early 1960's because the adhesive bond to the copper foil was attacked by the gold plating solutions used to electroplate gold contact areas on the printed wiring boards.